Integrally rotating turbo machinery and method and apparatus for achieving the same

ABSTRACT

A method of inducing compressive residual stress of blading members integrally formed with a rotor. The method includes using a caliper-type burnishing tool such that the caliper arms of the tool are contoured and oriented for placing the burnishing elements on opposite sides of the blading member to be treated. The blading member is burnished to introduce a pre-determined pattern of residual compressive stresses in a selected area of the blade.

This application is a divisional application of and claims benefit ofU.S. patent application Ser. No. 11/546,970, filed Oct. 12, 2006, whichclaims benefit of U.S. Provisional Application No. 60/726,038, filedOct. 12, 2005.

ACKNOWLEDGMENT OF FEDERAL GRANTS

The U.S. Government may have certain rights in this invention pursuantto contract number F33615-03-C-5207 awarded by the U.S. Department ofthe Air Force.

TECHNICAL FIELD

The present invention relates generally to rotating turbo machinery,and, more specifically, to an integrally bladed rotor or disk havingimproved fatigue performance, greater foreign object damage tolerance,and increased resistance to stress related failure mechanisms due to theintroduction of compressive residual stresses through burnishing and amethod and apparatus for producing the same.

BACKGROUND

The high vibratory and tensile stresses experienced by rotating turbomachinery in operation, particularly the blading members of the fan,compressor, and turbine stages in gas turbine engines, make suchcomponents susceptible to high cycle fatigue (HCF) and other stressrelated failure mechanisms such as stress corrosion cracking (SCC). HCFand SCC ultimately limit the service life of these components asprolonged exposure to such extreme operating conditions leads to thedevelopment of fatigue cracks in areas of the component subject to highoperational stresses. The fatigue life of a component is further limitedby the occurrence of foreign object damage (FOD). FOD locations act asstress risers or stress concentrators that hasten the development andpropagation of fatigue cracks. FOD, especially along the leading andtrailing edges of blading members, significantly reduces the servicelife of aerospace components.

The potentially catastrophic effects of HCF and FOD require thatfatigue-life limited components be periodically inspected for bothcracks and FOD. Any damage or cracking found during inspection isassessed and the component is retired from service due to the extent ofthe damage or else repaired and returned to service. The inspection ofparts and the retirement of parts from service adversely impacts bothflight readiness and maintenance costs of the aircraft.

Integrally formed components, such as rotors integrally formed withblading members, also known in the industry as blisks (bladed-disks),blings (bladed-rings), and IBRs (Integrally Bladed Rotors), incursignificant maintenance and repair costs due to stress related failuremechanisms and FOD. This is a direct result of their integral or unitarydesign as opposed to more traditional rotating turbo machinery, such asbladed rotors, where individual components of the construct, such asindividual blading members, can be separately removed and repaired orreplaced when damage is discovered or the component has reached itspre-determined service life.

FOD and stress related cracking in a single blading member of anintegrally formed component may directly impact the integrity of theentire component. Because the integrally formed blading members are notreadily removable or replaceable in the event of such damage, an entireintegrally bladed rotor may be withdrawn from service due to damageconfined to a single blading member. The repair and/or replacement ofsuch a complex component is expensive, both monetarily and from a flightreadiness perspective.

The need to replace or repair integrally bladed rotating turbo machinerymay be significantly reduced if the fatigue strength, FOD tolerance, andresistance to stress related failure mechanisms of new, serviced, andrepaired components can be improved or restored to the as-manufacturedcondition. Common methods of improving the fatigue strength and foreignobject damage tolerance of aerospace components include the introductionof residual compressive stresses in critical areas susceptible to damageand fatigue failure such as the edges and tips of blading members.Introducing compressive residual stresses improves the fatigueproperties and foreign object damage tolerance of both new and repairedblading members. This decreases operation and maintenance costs andincreases the flight readiness of the aircraft in which the component isemployed.

One method currently used to introduce compressive residual stresses inthe blading members of integrally bladed rotating turbo machinery islaser shock peening (LSP) as disclosed in U.S. Pat. No. 6,541,733. LSPuses a high power laser system to impart compressive residual stressesat discrete locations on both sides of the integrally formed airfoil orblading member. However, LSP processing each blade of an integrallybladed rotor is labor intensive, time consuming, and expensive.

Burnishing, also referred to as deep rolling, is an equally effective,less expensive, and more time efficient alternative to LSP for inducingcompressive residual stresses in the surface of a part. Burnishing,particularly ball burnishing as disclosed in U.S. Pat. Nos. 5,826,453,6,415,486, and 6,622,570, has been shown to effectively increase thefatigue strength and FOD tolerance of aerospace components, such asairfoils and turbine disks, and to substantially mitigate or eliminatestress induced failure mechanisms.

While burnishing is generally well suited for aerospace applications,the geometrical complexity and unitary design of some aerospacecomponents, such as integrally bladed rotors, does not readily permitthe use of current, commercially available burnishing tools to introducecompressive residual stresses in the individual, integrally formedblading members. As a practical matter, the complex shape of the bladingmembers and the narrow spacing between individual blading members of theintegrally bladed rotor does not provide adequate clearance to permitthe use of current tool designs to accomplish the introduction ofcompressive residual stress.

Accordingly, a need exists for an efficient and cost effective method ofimparting residual compressive stresses in the individual bladingmembers of integrally bladed rotating turbo machinery to either improveor restore the fatigue performance and/or resistance to stress relatedfailure mechanisms of the blading members thereof.

DISCLOSURE OF THE INVENTION

A rotor integrally formed with blading members for a turbine or turbomachinery having improved fatigue performance, FOD tolerance, andresistance to stress related failure mechanisms is produced byintroducing compressive residual stresses in the surface of theindividual blading members. The rotor is mounted on the worktable of aCNC machine tool. A caliper burnishing tool is positioned relative tothe individual blading member to be treated. The geometry of theburnishing tool is such that it can be positioned relative to theblading member to be treated without contacting or otherwise interferingwith adjacent blading members. The blading member is burnishedintroducing compressive residual stresses in the surface of the bladingmember. The burnishing tool is withdrawn and the rotor is rotated suchthat a subsequent blading member may be treated.

One embodiment of the present invention is a rotor integrally formedwith blading members having improved fatigue performance and increasedtolerance to FOD as a result of compressive residual stresses introducedin individual blading members by burnishing.

In another embodiment, the present invention is a method for improvingthe fatigue performance and FOD tolerance of a rotor integrally formedwith blading members by introducing compressive residual stresses in theblading members of the rotor by burnishing.

In another embodiment, the present invention is an apparatus forinducing compressive residual stresses in blading members integrallyformed with a rotor thereby improving the fatigue performance and FODtolerance of the rotor. The apparatus consists of two burnishingelements oriented in opposition to one another such that a bladingmember may be disposed therebetween. This facilitates the simultaneousintroduction of compressive residual stresses on both surfaces of theblading member. The burnishing elements are disposed in contouredcaliper arms that conform to the complex geometry of the blading memberand permit the in situ treatment of individual blading members of arotor.

Other aspects, advantages and embodiments of the invention will beapparent from the following description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings where:

FIG. 1 is a schematic illustration of a rotor integrally formed withblading members treated according to the current invention.

FIG. 2 is a schematic drawing of one embodiment of the burnishing toolthat is a subject of the present invention.

FIG. 3 is a schematic drawing of another embodiment of the burnishingtool that is a subject of the present invention.

FIG. 4 is a flow chart of the method for improving the resistance tostress related failure mechanisms of a rotor integrally formed withblading members.

FIG. 5 is a schematic drawing illustrating a preferred embodiment of themethod of the current invention.

FIG. 6 is a schematic drawing showing a prior art caliper-typeburnishing tool.

FIG. 7 is a schematic drawing showing the apparatus of the presentinvention having contoured interior surfaces.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention relates to rotating turbo machinery integrallyformed with blading members having improved or restored resistance tofatigue, FOD, and stress related failure mechanisms. As shown in FIG. 1,the rotor 100 consists of a central disk 102 integrally formed withblading members 104 spaced about the periphery and extending radiallyoutward from the center 106 of the rotor. The blading members 104 aredefined by a leading edge 108, a trailing edge 110, a tip 112, apressure side 122, and a suction side 124.

Using a burnishing process, compressive residual stresses are induced inthe blading members 104 of the rotor 100 in continuous compressive zones114, on both the pressure side 122 and the suction side 124, along theleading edge 108, trailing edge 110, or tip 112, and combinationsthereof, in locations on the blading member where damage and failuresare known to occur. The continuous compressive zones 114 whereincompressive residual stresses are induced are identified by operationalexperience of the component, testing, or mathematical modeling of theresidual and applied stress state of the component, and combinationsthereof. The location and shape of the continuous compressive zones 114are designed to offset areas of residual tensile stress and high appliedstress or to mitigate FOD known to occur in the general area throughoperational experience. In one embodiment, the compressive residualstresses extend substantially through the thickness of the bladingmembers from both the pressure side 122 and the suction side 124 suchthat the entire cross section of the blading member is under compressivestress.

In a preferred embodiment of the invention, the compressive residualstresses induced in the blading members 104 of the rotor 100 have anassociated cold work on the order of less than about 5%, preferably lessthan about 3.5% in order to create thermally and mechanically stablecompressive residual stresses.

In another preferred embodiment of the invention, the continuous zonesof compressive residual stress 114 have an improved surface finish as aresult of the burnishing method used to induce the residual compressivestresses, which, in turn, benefits the aerodynamic efficiency of theblading members 104. The improved surface finish is in the range ofabout 5 μin. to 20 μin., depending on the alloy from which the bladingmembers 104 are manufactured and the characteristics of the burnishingtooling used. Common alloys used in this type of aerospace applicationinclude, but are not limited to, titanium alloys, stainless steels, andnickel-base alloys.

For purposes of the current invention, prior to burnishing, the rotor100 may be in the as manufactured condition, such as a new rotor or arotor at an intermediate manufacturing step or a rotor with nooperational service time. Alternatively, the rotor 100 may have beenpreviously fielded and therefore subject to reduced performance due tooperational stress and/or FOD.

A preferred embodiment of the apparatus of the present invention isshown in FIG. 2. The apparatus 200 is a caliper-burnishing toolconfigured as a second-class lever with a first caliper arm 202 and asecond caliper arm 204 in pivotal opposition to one another about apivot point 206. The anterior surfaces of the first caliper arm 202 andthe second caliper arm 204 are shaped to facilitate insertion of eachcaliper arm between the blading member being treated and adjacentblading members. In addition, the interior surfaces of the first caliperarm 202 and the second caliper arm 204 are contoured to conform to thecomplex surface geometry of the blading member being treated.

The benefit of the curved interior surfaces of the first caliper arm 202and the second caliper arm 204 is shown in FIG. 6. FIG. 6 shows aconventional straight-armed caliper burnishing tool 602 positionedrelative to an airfoil 600 shown in cross section. The airfoil 600 has acomplex, curved geometry in the chord-wise direction. The burnishingelements of the conventional straight-armed caliper tool 602 are unableto contact all points along the curved surface of the airfoil 600without the caliper arm also contacting the airfoil 600 and potentiallydamaging the airfoil 600. As shown in FIG. 7, the apparatus 200 of thesubject invention is shown wherein the caliper arms 202 and 204 comprisecurved interior surfaces 604 that operate to facilitate the treatment ofthe entire curved surface of the airfoil 600 by maintaining sufficientclearance between the caliper arms 202 and 204 and the airfoil 600thereby reducing the risk of damage to the airfoil 600.

Referring again to FIG. 2, the pivot point 206 is attached to a base208, which, in turn, is attached to a tool holder 210 that facilitatesinsertion of the apparatus 200 into the chuck of a CNC machine tool orother positioning device including, but not limited to, roboticpositioning devices.

The first caliper arm 202 and the second caliper arm 204 are pivotedabout the pivot point 206 by an actuator 212 located below the pivotpoint 206. The actuator 212 mechanically links the first caliper arm 202and the second caliper arm 204 via the linkage 214. The actuator 212 maybe selected from the list including, but not limited to, hydrauliccylinders, pneumatic cylinders, electromagnetic solenoids, andmechanical actuators such as springs.

The first caliper arm 202 and second caliper arm 204 each haveburnishing elements 216 oriented in opposition to one another on theinterior surface 604 (FIG. 7) of each caliper arm and disposed withinsockets 222 distally located from the pivot point 206. Preferably theburnishing elements 216 are in the form of burnishing balls forproviding single point burnishing and the sockets 222 are sized andshaped, such as being spherical, to receive the burnishing elements 216.In a preferred embodiment, the sockets 222 are provided with a constantvolume flow of fluid via fluid supply lines 218. The constant volumeflow of fluid serves to suspend each of the burnishing elements 216 overthe surface of their respective socket 222 on a thin film of fluidthereby creating a hydrostatic bearing.

In another preferred embodiment of the present invention the apparatus200 is configured as a first-class lever, as shown in FIG. 3. In thisconfiguration the pivot point 206 is attached to a yoke 224 such thatthe first caliper arm 202 and the second caliper arm 204 may pivot withrespect to each other and the yoke 224. The yoke 224 it attached to thebase 208, which in turn, is attached to a tool holder 210.

Referring again to FIG. 2 to illustrate the operability of a preferredembodiment of the apparatus, the apparatus 200 is positioned around aworkpiece, such as the blading member 104 of an integrally bladed rotor100 as shown in FIG. 1, so that the first caliper arm 202 and the secondcaliper arm 204 are positioned relative to the surfaces of the workpiece(such as the airfoil 600 as shown in FIG. 7) to be treated. A constantvolume supply of fluid is provided to the sockets 222 such that theburnishing elements 216 are fluidly supported over the surface of thesocket. The actuator 212 is then activated retracting the linkage 214and advancing the first caliper arm 202 and the second caliper arm 204towards one another. The workpiece is impinged between the burnishingelements 216. With the burnishing elements 216 in contact with theworkpiece, the caliper tool 200 is moved along the surface of theworkpiece imparting compressive residual stresses on both sides of theworkpiece and substantially through the cross sectional area (thicknessT, FIG. 7) of the workpiece. The caliper tool 200 may be advanced alongthe workpiece in a predetermined pattern thereby imparting the desiredamount of residual compressive stress. The depth and magnitude of theinduced compressive residual stress relative to locations on theworkpiece is preferably precisely and continuously controlled byadjusting the force by which the actuator 212 impinges the burnishingelements 216 against the workpiece as the caliper tool traverses thesurface of the workpiece in a predetermined pattern under CNC control.

It should be obvious to one skilled in the art that the burnishingelements 216 may also be pinch-peening elements, indenting elements,coining elements, or roller elements all of which may be used to induceresidual compressive stress in the surface of a blading member.

The method of the present invention may be carried out in a series ofsteps as shown in FIG. 4. In a first step 401, the rotor integrallyformed with blading members is mounted on a fixturing device. As shownin FIG. 5, the fixturing device 504, which is positioned on the x-ytable (not shown) of a CNC machine tool such as a vertical mill, permitsthe precise positioning of the rotor 502 with respect to acaliper-burnishing tool 506 held in the chuck 508 of the machine tool.More specifically, the fixturing device 504 allows for the rotation ofthe rotor 502 about its normal axis of rotation (the z-axis of FIG. 5)while simultaneously facilitating the rotation of the combination of therotor 502 and the fixturing device about the x-axis as indicated in thefigure. Orientation of the fixturing device 504 and rotor 502 in the x-yplane relative to the caliper-burnishing tool 506 is controlled by thex-y table of the CNC machine tool on which the fixturing device 504 ismounted. The relative positioning of the caliper burnishing tool 506 andthe fixturing device 504 along the z-axis is controlled by advancing andretracting the chuck 508 of the CNC machine tool in the direction of thez-axis as well as rotating the fixturing device 504 in the y-z plane.Rotation of the rotor 502, rotation of the fixturing device 504 in they-z plane, and positioning of the caliper-burnishing tool 506 areaccomplished under CNC control.

In a second step 402, a computer program is used in conjunction with theCNC controls of the machine tool to automatically carry out thetreatment operation on each of the integrally formed blading members ofthe rotor. Steps 403 through 408 are accomplished under computer controlusing CNC code. In a first program step 403, the program rotates therotor into position such that the proper profile of the blading memberis presented to the caliper tool to facilitate treatment of anindividual, integrally formed blading member. Referring again to FIG. 5,this is accomplished by rotating the fixturing device 504 in the y-zplane and rotating the rotor 502 about its axis of rotation until theproper profile of an individual blading member is presented to thecaliper-burnishing tool 506. The proper profile is obtained when thestacking axis of the individual integrally formed blading member isaligned with the z-axis of the machine tool.

Returning to FIG. 4, in the next program step 404, the caliper tool islowered into position and inserted between adjacent blading members suchthat the caliper arms of the tool are located on either side of theblading member being treated. This step is illustrated in FIG. 5. Thespecific shape of the caliper arms permits the tool to be positioned inthis manner without damaging or otherwise interfering with adjacentblading members while the contoured interior surfaces of each caliperarm permit the tool to be positioned in close proximity to the surfaceof the blading member being treated without damaging the blading member.

In a third program step 405 following insertion of the caliper tool, theburnishing process begins. It is during this step that the caliper toolcloses around the blading member such that the burnishing elements ofeach caliper arm are in contact with opposing surfaces of the bladingmember. The hydraulic cylinder is actuated to impinge the burnishingelements against both sides of the blading member, thereby impartingresidual compressive stresses. The force exerted by the tool against thesurface of the blading member is regulated by the pressure of hydraulicfluid supplied to the hydraulic cylinder, which, in turn, is regulatedby the CNC program. This permits the pressure exerted against thesurface of the blading member, and therefore the induced residualcompressive stress, to be precisely controlled and adjusted inconjunction with the position of the treatment apparatus.

In a subsequent program step 406, a predetermined residual compressivestress pattern is imparted in the surface of the blading member bymoving both the caliper tool and the rotor relative to one another in acontinuous operation. During this step the CNC positioning controls areutilized to precisely control the positioning of both the caliper tooland the rotor. The burnishing elements, which are essentiallyhydrostatic bearings, in combination with the precision CNC programcontrols permit the caliper tool to smoothly and accurately follow theunique contours of the individual blading member thereby producing thedesired residual compressive stress pattern.

In the fifth program step 407, following the treatment of a singleblading member, the tool is withdrawn from the rotor. In a final programstep 408, the process is repeated until the desired residual compressivestress patterns have been induced in each of the integrally formedblading members on the rotor.

In a preferred embodiment, the compressive stress pattern impartedprovides a compressive residual stress zone that extends along a portionor extends substantially along the entire perimeter, such as the entireleading edge 108, trailing edge 110, and tip 112 (FIG. 1) and inwardstowards the center C of the blading member 104.

Referring to FIG. 1, another embodiment of the invention is an article100, such as a rotor, having a plurality of blading members 104 eachhaving a pressure side 122, a suction side 124 in opposition to oneanother and a perimeter defined by a leading edge 108, a trailing edge110, and a tip 112. At least one blading member 104 has continuous zonesof compressive residual stress on its pressure side 122 and its suctionside 124 such that the continuous zones of compressive residual stressextends substantially along the perimeter and extending inwards in adirection generally towards the center C. It should now be apparent thatthe article formed such as by the method and apparatus described above,unlike articles formed using laser shocking methods or shot peeningmethods that produce a plurality of discrete points of compression, hascontinuous zones of compressive residual stress thereby improving thesurface finish of the part and possibly reducing the potential forstress induced damage.

It should be apparent to one skilled in the art that the method andapparatus described herein may also be used to introduce compressiveresidual stresses in blading members removably connected to a centralrotor such as that commonly used in rotating turbines and turbomachinery.

A principle advantage of the apparatus is the ability to introducebeneficial compressive residual stresses in the individual bladingmembers of an integrally bladed component using a mechanical surfacetreatment such as burnishing. The configuration of the apparatus permitstreatment of individual blading members without interfering with ordamaging adjacent blading members. Further, the contoured interiorsurfaces of the caliper arms permit the treatment of curved or complexairfoil surfaces without the risk of damage to the individual airfoilbeing treated.

Another advantage of the present invention is a low cost method ofimproving the resistance of an integrally bladed rotating component tostress induced failure mechanisms such as FOD, fatigue, and stresscorrosion cracking through the introduction of beneficial compressiveresidual stresses by a mechanical surface treatment such as burnishing.The method reduces overall manufacturing costs compared to currentlyemployed methods of inducing compressive residual stresses in theblading members of integrally formed rotating components and improvesmanufacturing throughput.

Another advantage of the present invention is an integrally bladedrotating component with improved resistance to stress related failuremechanisms such as FOD, fatigue, and stress corrosion cracking throughthe introduction of beneficial compressive residual stress by mechanicalsurface treatment such as burnishing.

While the method and apparatus described herein constitute preferredembodiments of the invention, it is to be understood that the inventionis not limited to the precise method and apparatus, and that changes maybe made therein without departing from the scope of the invention whichis defined in the appended claims.

1. A method of burnishing blading members affixed to a central rotor toinduce compressive residual stress, the method comprising: positioningthe rotor to permit the engagement of a burnishing tool with anindividual blading member; engaging the burnishing tool with the bladingmember so as to simultaneously induce a desired compressive residualstress distribution in both sides of the blading member; traversing theburnishing tool over the surface of the blading member to induce acontinuous zone of compressive residual stress in the surface of theblading member; disengaging the burnishing tool from the blading member;rotating the rotor; repeating the process until compressive residualstress has been induced in each blading member affixed to the rotor. 2.The method of claim 1 wherein the continuous zone of compressiveresidual stress have a surface finish of about 5 μin. to about 20 μin.3. The method of claim 1 wherein the blading members are integrallyformed with the rotor.
 4. The method of claim 1 wherein the caliper-typeburnishing tool comprises a pair of caliper arms in pivotal engagementwith each other, each caliper arm having an interior surface and anexterior surface, the interior surface being contoured to conform to thesurface of the blading member and having a burnishing element locatedthereon.
 5. The method of claim 1 wherein the compressive residualstress extends substantially through the thickness of the bladingmember.
 6. The method of claim 1 wherein the compressive residual stresszone extends substantially along the perimeter of the blading member andinwards, towards the center of the blading member.
 7. An articleintegrally formed with blading members comprising: a rotor; a pluralityof blading members attached to said rotor, each said blading memberhaving a pressure side and a suction side in opposition to one anotherand a perimeter defined by the leading edge, trailing edge, and tip ofsaid blading member; wherein said blading members have continuous zonesof compressive residual stress on said pressure side and said suctionside of each said blading member, said continuous zones of compressiveresidual stress extending substantially along said perimeter of saidblading member and extending inwards, towards the center of said bladingmember; wherein said continuous zones of compressive residual stresshave an improved surface finish.